Plate Tectonics

The Plate Tectonic Theory

The plate tectonic theory states that the crust of the Earth is broken into several

large sections known as crustal or

lithospheric plates.

It is theorized that these plates are

dynamic and move continually. The

interaction between plates produces

changes on Earth’s surface such as,

volcanoes, mountains, and earthquakes.

The interface between two or more plates is

known as a boundary.

NOTE-- Divergent Boundaries are in blue, Convergent Boundaries are in Red.

Why do the crustal plates move?

Crustal plates may move away from one another, towards one another, or slide against one another. In addition, crustal plates may rise above (uplift) or descend

(subsidence) within the Earth.

It is theorized that crustal plates move due to the convection currents of heat within the asthenosphere.

Tectonic Plates of the World

RT = Pg. 5

“Which tectonic plate is located at

20˚N, 160˚W?”

PACIFIC PLATE

“What type of boundary is the

Mid-Atlantic Ridge?”

DIVERGENT PLATE

BOUNDARY

Divergent Plate Boundary

A divergent plate boundary is where two plates separate from one another. This separation leads

to the rise of magma and igneous intrusions/extrusions at the point of separation.

As the plates diverge, magma and

lava are forced upward due to

convection in the asthenosphere.

The molten rock will cool to form new

crust.

Soon thereafter, new magma will be

forced upward and will force the new crust to split and diverge as well. When a divergent plate boundary occurs under an ocean, it is

also referred to as sea-floor spreading.

Young Crust

Older Crust

Divergent Plate Boundary

When a divergent plate boundary occurs under a continent, it is also referred to as

rift valley.

Dynamic activity associated with a divergent plate boundary includes:

1. Many shallow earthquakes2. Faulting (sudden uplift of

small portions of the crust)3. Volcanic activity4. The creation of a ridge of

volcanic mountains (ocean only).

Convergent Plate BoundaryA convergent plate boundary is where two

plates converge or collide with one another. Convergent plate boundaries can take place in

three ways:

OCEANIC-OCEANIC CRUST CONVERGENCE

CONTINENTAL-CONTINENTAL CRUST CONVERGENCE

OCEANIC-CONTINENTAL CRUST CONVERGENCEOCEANIC-CONTINENTAL CRUST

CONVERGENCE

CONTINENTAL-CONTINENTAL CRUST CONVERGENCE

OCEANIC-OCEANIC CRUST CONVERGENCE

Convergent Plate Boundary: SUBDUCTION ZONE

When two plates converge, the denser of the two plates will move and sink under the less dense

plate. This process is known as subduction.

A subduction zone is an area of tremendous heat, pressure, and dynamic activity. Many

features are associated with convergent plate boundaries including:

1. Volcanic activity2. Deep earthquake activity3. Regional metamorphism4. Orogeny – Mountain-building forces5. Deep ocean trenches6. Deposition basins

OCEANIC-CONTINENTAL CRUST CONVERGENCE

CONTINENTAL-CONTINENTAL CRUST CONVERGENCE

OCEANIC-OCEANIC CRUST CONVERGENCE

Transform Plate Boundary A transform plate boundary is where

two plates slide past one another.

Dynamic activity associated with a transform plate

boundary includes:

1. Many earthquakes2. Faulting (sudden uplift of

small portions of the crust)

San Andreas Fault, California: Transform plate boundary between the North

American Plate and Pacific Plate

Evidence to Support the Plate Tectonic Theory Appearance of Continents

The outline of many present day

continents seem to fit together like pieces of

a puzzle.

Evidence to Support the Plate Tectonic Theory Landmass Features/Fossil Record

Many rock assemblages are identical in formation and age on different continents.

Fossil evidence supports the notion that the continents were once connected.

Evidence to Support the Plate Tectonic Theory Age and Heat Index of Ocean Rock

In a divergent plate boundary, molten rock rises to the surface and cools to

become new crust.

This newly formed crust is continually being replaced by new molten rock.

Older crust diverges and is forced to move away from the area where new molten rock will form into new crust.

This image is taken of the Mid-Atlantic Ridge – a divergent plate boundary.

Each color represents rocks of similar age.

The red colored rock represents the youngest rock (closest to the source of divergence). The blue colored rock represents the oldest rock.

Evidence to Support the Plate Tectonic Theory Magnetic Polarity

Earth’s magnetic field has reversed its polarity many times in Earth’s history.

Currently magnetic north is close to Earth’s North Pole. Yet, in the past,

magnetic north was close to Earth’s South Pole.

The nature and frequency of these reversals of magnetic polarity are not fully

understood.

When molten rock cools at the site of divergence, the minerals within the molten rock begin to crystallize. Many minerals found within igneous rock are magnetic. These minerals will align themselves in

the direction of magnetic north.

If there is a reversal in magnetic polarity the minerals with igneous rock will align in

an opposite pattern.

These patterns suggest some plates are moving away from one another.